Fouling of membranes used for ultrafiltration and reverse osmosis by bulky and generally water-insoluble or hydrophobic molecules has been noted in the literature. The types of molecules which cause difficulty include, generally, any large molecule, whether having a cationic or anionic charge, amphoteric, nonionic, or hydrophobic. Virtually all oils, greases and waxes, proteins, sticky and gelatinous materials, and some larger scale-forming cations can cause difficulty. Such fouling greatly reduces the efficiency of the membrane and the separation process for which the membrane is used. Accordingly, the use of ultrafiltration and reverse osmosis for the separation or concentration of materials which include compositions such as greasy materials has been greatly restricted or entirely avoided. Prior to the present invention, no satisfactory method of avoiding the consequence of fouling by bulky materials has been known. In particular, the application of ultrafiltration and reverse osmosis processes to the treatment of sewage has been restricted because of the grease content of the sewage. Even as little as 2 ppm hydrophobic or greasy materials or other large molecules in the sewage has resulted in fouling of the membrane and breakdown of the separation process in a typical sewage containing total organics of about 300 ppm.
So-called "dynamic membranes" have been known prior to the present invention. A "dynamic membrane" may be defined as a microporous layer deposited on a porous substrate during or prior to use in a separation process by pressure through the microporous layer. A dynamic membrane is usually particulate and easily removed from the porous substrate.
Dynamic membranes are discussed by Savage et al. in "Hyperfiltration of Plant Effluents," Water & Sewage Works, V. 116, No. 3, (March, 1969), pp 102-106, and by Johnson et al. in U.S. Pat. No. 3,431,201.
A filter aid composition comprising two adsorbents, activated carbon and silica gel, is disclosed in French patent 2,005,394.
The formation of a "gel layer" on top of an ultrafiltration membrane has been studied by Baker and Strathman, "Ultrafiltration of Macromolecular Solutions with High-Flux Membranes," J. App. Polymer Sci., V. 14, (1970), pp 1197-1214, and others. A number of materials such as various hydrous oxides and hydrolyzable metals, as well as various polyelectrolytes have been used to form dynamic membranes on rigid porous substrates. See, for example, Kuppers et al. "Filtration of Organic Solutes by Dynamically Formed Membranes," Separation Science, 2(5), (November, 1967), pp 617-623, wherein the use of organic polyelectrolytes, zirconium oxide, and bentonite is separately discussed, Shor et al. "Concentration Polarization in Tubular Systems with Dynamically Formed Membranes," I & EC Fundamentals, V. 7, (February, 1968), pp 44-48, discussing the deposition of colloidal hydrous zirconium oxide on a porous carbon tube, and Johnson et al. "Hyperfiltration Studies XIV - Porous Tubes Precoated with Filter Aids as Supports for Dynamically Formed Membranes," Desalination, V. 5, (1968), pp 359-369.
Work similar to the above is disclosed in U.S. Pat. Nos. 3,449,245, 3,503,789 and 3,413,219.
Brownscombe et al., U.S. Pat No. 3,331,772, use bentonite treated with sodium polyacrylate as a preferred combination of clay and water-soluble polymer to form a precoat on a porous substrate and to repair semipermeable membranes.
Flowers et al., U.S. Pat. No. 3,457,171, disclose the use of graphitic oxide, with and without a binder, to form a coating on a semipermeable membrane.
Kollsman, U.S. Pat. No. 3,462,362, deposits on a porous substrate two layers of polyelectrolytes having opposite charges. Montmorillonite and diatomaceous earth are among the materials used.
Waste products of various types, such as pulp mill wastes, have been treated by permeability separation techniques. See, for example, Wallace et al. U.S. Pat. No. 3,528,901 which discloses the use of reverse osmosis to treat effluent from a paint plant. Activated sewage sludge has also been concentrated or dewatered -- also see Water & Wastes Engineering, (January, 1970), pp 51-52, and Hinden et al. "Organic Compounds Removed by Reverse Osmosis," Water & Sewage Works, V. 116, No. 12, (December, 1969), pp 466-470. Secondary sewage effluent is treated in Marcinkowsky et al. U.S. Pat. No. 3,537,988. Primary effluent is used in the Savage et al article mentioned previously. Attempts to treat raw sewage with reverse osmosis are reported in an FWQA research report entitled "Reverse Osmosis Renovation of Municipal Wastewater"; metaphosphates and polyelectrolytes are used as aids.
So far as I am aware, no natural absorbent such as kaolin clay, bentonite, montmorillonite, hectorite, diatomaceous earth or the like has been used to form a dynamic membrane on top of a pre-formed, permanent semipermeable membrane which, in turn, is on top of a porous substrate, in order to minimize fouling of the permanent membrane.
Such absorbent substances are, of course, commonly used for water treatment (see Hronas U.S. Pat. No. 3,066,095 as an example). They have also been used in processes involving activated carbon in the treatment of water, such as in Rice et al. U.S. Pat. No. 3,252,899. Sewage treatment processes which include or mention the use of both activated carbon and clay absorbents include U.S. Pat. Nos. 3,171,802, 3,142,638, and 3,337,454.